THX recommends filling Speaker cabinet with poly-batting?

[djk]

" Adding polyfil in the cabinet may increase the bass response. "

Please explain how, and in great detail.

"This is
an old trick with subwoofers to make it behave as a bigger box."

A bigger box gives an alignment with a lower Qtc.

A box with a Qtc=.707 is called a Butterworth, Qtc=.577 Bessel, etc.

The Qtc=.577 has 1.765 dB less bass than the Qtc=.707

Using a fiberous tangle (fiberglass, poly fiberfill, wool, etc) reduces the amount of bass by friction.

In some cases this may improve the perceived sound quality. By reducing the output at the box resonance the bass may sound 'tighter' or 'faster', but make no mistake, there will be less bass.

"make it behave as a bigger box."

This has been repeated so many times out-of-context that I am sick and tired of it.

[derrickdj1]

In general, a subwoofer box that would require polyfill would be a subwoofer box
that is close to, or smaller than the manufacturer's minimum recommendations for
internal volume. Polyfill will make a subwoofer perform as if it were in a
larger enclosure by slowing sound waves as they pass through the polyfill. The
physical internal volume of the subwoofer box remains the same, yet the
effective internal volume changes when polyfill is added.

An isothermal process occurs once the polyfill has been added. As the air passes through the polyfill, the fibers wiggle and cause some of the energy created by the airspring to be dissipated as heat. This heats the surrounding air molecules warmer, causing the air to become less dense. Being that sound passes easier through a denser medium, the speaker interacts with your enclosure as if it is larger than it actually is. The effective increase in enclosure size can be as much as 40%!

This has some very obvious benefits that are inherent of a larger enclosure. Firstly, it becomes more efficient (a larger enclosure is always more efficient than a smaller one for any given driver). Second, the f3 (or the frequency at which SPL is down by 3dB) will be lower, providing a little bigger bottom end. While these are both great advantages, they decrease the effective damping of the speaker as well, meaning the speaker can be more likely to bottom out or over-excurt itself. Naturally, this is speaker, frequency, and power dependent. If used in a ported enclosure, you will also see the Fb (or the resonant frequency of your port) drop lower. As a result, it will extend the ultra low frequencies of the subwoofer and will
flatten the frequency response curve. However, the mid to upper frequency output
may drop.

http://forum.blu-ray.com/showpost.php?p=1376150 has a nice article on the use of polyfil and it's benefits and lack of benfits in various applications.

[Don Richard]

From somewhere in cyberspace:

"The particulars of fiber stuffing are pretty interesting: The air inside your enclosure actually heats up as your woofer moves, making the
air stiffer. (I'm absolutely serious.) When the enclosure is stuffed with fiber,
the fibers wiggle, dissipating some of the heat and making the system work as
though the box were larger. Theoretically, your woofer/box bass system can act
like a system that's a maximum of 40 percent larger when you've latched onto the
right stuffing recipe in other words, if you have an enclosure that offers 1
cubic foot (1 ft³ ) of internal volume, in a perfect world a good stuffing job
will make it perform like an enclosure that offers 1.4 cubic feet of internal
volume.

There are three types of stuffing that are commonly used for this
purpose: fiberglass insulation, long-fiber wool, and polyester fiberfill.
Fiberfill is the best choice because it doesn't come loose and fly around and
irritate your skin or lungs like fiberglass, it works as well as either of the
others, it's a lot cheaper than wool, and moths hate it. I recently bought five
20-ounce bags of it at $1.99 a pop (a total of 6.26 pounds for $9.95) at
Minnesota Fabrics; that turns out to be about $1.60 a pound. You should be able
to find some at any fabric store or in the bedding section at friendly stores
like K-Mart or Home Depot.

To evaluate the effectiveness of box stuffing,
I used an MLSSA analyzer to measure the impedance of three enclosures
5.l-cubic-foot sealed, 1.4-cubic-foot sealed, and 1.4-cubic-foot ported (the
port measured 3 inches in diameter and 6 inches in length) with various
densities of stuffing. For the sealed boxes, I was able to determine the
effective box size as enhanced by the stuffing using the system's
resonant-frequency and Qes values. For the ported box, I compared the tuned
frequency of the empty enclosure to the tuned frequency of the stuffed
enclosure, using the Speak for Windows computer program; this enabled me to find
the effective box size that fit the actual resonant frequency I'd measured.




In
each case, the news was good make that very good. With all three boxes, I
enjoyed roughly 25 to 35 percent of "space gain" by using stuffing at a rate of
1 to 1.75 pounds per cubic foot of internal volume.

When making system
performance predictions, be aware that the Qes figure and, therefore, the Qts
figure of the sealed boxes dropped. And with the ported box, the peak of the
impedance curve on the lower side of the tuned frequency became heavily damped
below the box's point of resonance. I also found that there is such a thing as
too much of a good thing: System resonance (Fsb) rises again, beginning with
densities of around 1.5 pounds of stuffing per cubic foot of box volume; this
happens because the fibers are jammed so tightly together that they stop
wiggling and, consequently, stop dissipating heat.

I also found that
stuffing gets less effective as box size increases. The morale: The bigger your
box is, the harder it is to fool your woofer."

[muel]

[:|]

[djk]

[Don Richard]

Completely irrelevant. You have changed box sizes in a simulation program by a factor of nearly 2:1 ( a factor of 1.4:1 is the theoretical maximum ) and added "fill = heavy", whatever that means, to the smaller box in order to draw a curve that supports your view. Poly fill changes the apparent size of a slightly undersized box when added in amounts of 1 lb. per cu. ft. to 1.6 lb. per cu. ft. Excessive amounts of fill overdamp the box. And of course a larger box is generally better for bass extension.

[DavidF]

No material in ealier Heresy models. It is a relatively small enclosure to be able to support standing waves but still known to be "lively". The second generation included a sheet of foam bent in semi circle around the woofer. Not sure about the 3rd generation.

[Chris A]

Using a fiberous tangle (fiberglass, poly fiberfill, wool, etc) reduces the amount of bass by friction.

This brings up an interesting side discussion: has anyone seen or calculated the SPLs inside the box when playing at or close to its rated output? Does hornresp, etc. need to be corrected for non-ideal gas performance and/or water vapor condensation effects at those calculated or measured SPLs?

Chris

Edit: The reason for interjecting this topic is that by placing a lot of filling material in the box, you are changing non-linear characteristics of the woofer/box/air-sping system. I don't see anyone talking about those.

[djk]

"Completely irrelevant. You have changed box sizes in a simulation program by a factor of nearly 2:1 ( a factor of 1.4:1 is the theoretical maximum ) and added "fill = heavy", whatever that means, to the smaller box in order to draw a curve that supports your view. Poly fill changes the apparent size of a slightly undersized box when added in amounts of 1 lb. per cu. ft. to 1.6 lb. per cu. ft. Excessive amounts of fill overdamp the box. And of course a larger box is generally better for bass extension. "

Completely obtuse?

A box with a Qtc=.707 was designed, same box was filled and the new Qtc noted, then a new box with the same Qtc as the filled box was designed.

The curves show what you would expect to see: filling the box reduces the Qtc and reduces the amount of bass slightly, and a non-filled box would need to be much larger for the same Qtc, but would have more output.

[Don Richard]

The curves you provided show that the box with fill is overstuffed and overdamped. Run the curve with less fill if your simulator will allow that.

"I also found that there is such a thing as too much of a good thing: System resonance (Fsb) rises again, beginning with densities of around 1.5 pounds of stuffing per cubic foot of box volume; this happens because the fibers are jammed so tightly together that they stop wiggling and, consequently, stop dissipating heat."

The proper amount of fill will not raise the resonant frequency. Your sim data shows the resonant frequency increases from 55.77 Hz to 62.16 Hz. Excessive fill is what is causing that.

[leftwinger57]

Just asking. Instead of poly fill could you use regular pink corning wall insulation? Or we're talking apples and oranges w/ fur.

[Don Richard]

Different types of fill will have different characteristics. Poly fill and fiberglass fill will give different results for the same weight of fill. A different amount of poly in the same box will give different results. Different types of fiberglass will also give different results. The best way is to stuff, then measure.

http://web.archive.org/web/20041027051204/http://www.integracaraudio.com/caraudio/resources/fiberfill/

[ivanhurd]

All I know is that people have done this with car subs for years. No reason it shouldn't provide the same help in an in home sub.